Every winter, like clockwork, the flu returns. It inf­ects millions of ­us -- about 5 to 20 percent of the U.S. population alone, according to the Centers for Disease Control and Prevention (CDC) [source: CDC]. Health Canada estimates that 10 to 25 percent of Canadians get the flu each year [source: Health Canada]. It leaves us sniffling, sneezing, coughing, achy and generally feeling miserable for anywhere from a few days to a few weeks.

Although most of us think of the flu as a mild annoyance that we have to deal with each winter, it can actually be a very dangerous disease. The CDC estimates that in the United States alone, more than 200,000 people are hospitalized­ with the flu or with flu-related complications each year, and more than 36,000 people die from it [source: CDC]. Around the world, the flu kills between a quarter of a million to half a million people a year [source: WHO].

­In this article, we'll find out how people get the flu, what the symptoms are and how to protect yourself.

Every species on Earth, from the majestic humpback whale to the bacteria happily living in your gut, has a special role to play within a defined ecosystem. Can organisms ever trade their existing niches for new ones?

Imagine a time in Earth's far-flung past when organisms didn't swim through the oceans or populate the lands. Even the mighty dinosaurs hadn't arisen. Will the planet ever return to that lifeless state?